In writing a novel, often flashbacks are difficult to manage: if too long and detailed, they can appear as second class (at worst, aborted) plots whose only task is to sustain the main plot line; if too short, they can create unwelcome confusion to the overall story flow.

2 Answers
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Vary their length and use them more often. If you think of flashbacks more like memories of varying length, it becomes apparent that they can be as short as one sentence or as long as an entire chapter. Humans are constantly bombarded with moments of past experience. Some are just a single second of time and others span whole years. Everything reminds someone of something else, so take advantage of this common human experience. Among other things, this technique makes it less obvious to the narrator when you are making an extended jump in time.

Have a flashback appear due to a logical trigger. In Stuart Dybeck's short "Pet Milk" the entire story is a flashback of a lost love triggered by the sight of milk swirling in coffee. The coffee reminded him of his love's favorite drink at a restaurant they always used to frequent, which leads to the intense memory of their last meeting. So using trick ties like this can make the flashback's surfacing seem more warranted/realistic and less like a tool to get across backstory.

A flashback can also appear more warranted if information communicated within it is pivotal to (or informs) the situation at hand.

Use flashbacks during a moment of inactivity when you would imagine the character logically calling up that memory. For instance: walking/driving/riding a bike/performing some menial, rote task/waiting a long time for whatever reason/etc. This not only makes the flashback seem more natural and less jarring, but the reader is more willing to "go along" with an extended flashback if there's not some moment of action/activity "waiting on another line."

The best advice I ever read regarding flashbacks was from the excellent book, "How To Write A Damn Good Novel" by James N. Frey. He commented that flashbacks are very much overused and misunderstood, and speculated that writers often use them because they get anxious about the characters they themselves have created, and are trying to avoid conflict.

At any rate, the advice he gave regarding the use of flashbacks is this: they are necessary "if your character is about to be plunged into a situation in which he will act contrary to the way he has been acting up to that point in the story ... In other words, the antecedent action must be relevant to the present story." He goes on to say that a flashback may also be necessary if your character in the present is quite loathsome, and you want to try and put him/her in a better light.

This always made the most sense to me, as well as his caveat: always "ask yourself if you can make the same impact on your reader through conflict in the now of the novel." I must admit, ever since, I've tended to try and avoid flashbacks unless absolutely necessary.

To be fair the question is vague. If it means "When should a flashback be used?" it does. If it means "I'm using one, how should I go about it?" it doesn't. Still, the awesomeness of the answer in application to case one remains undiminished +1
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One MonkeyMar 28 '11 at 9:53